If the mega-threads bother you, why not avoid them?  There's already an option in your forum settings to not display signatures if they are an annoyance to you.  I think the forum is happy that these mega-threads exist as they drive user interaction and inflate statistics so that the forum looks more popular.  This can help with the valuation of the forum or raising funds through advertisements or even ranking among websites.  I don't think there's really a good reason to start treating long threads differently.  Especially when the option I mention exists for users.